The present invention relates to a process for writing with updating and reading of informations on a non-erasable support, medium or carrier organized into sectors. Non-erasable supports are more particularly used for the bulk storage or filing of voluminous informations, because they are not very expensive. These supports are increasingly used in the bulk storage of files and digitized documents.
The digital optical disk constitutes an example of a non-erasable support organized into sectors. On such a support it is possible to write an information at any time on a random sector, but it is not possible to subsequently modify this sector. Such a support is said to be of the WORM type (write once read many, i.e. single writing and multiple reading). These WORM-type supports are very suitable for the bulk storage of data not subject to any changes. However, even in bulk storage or filing applications, it is necessary to keep certain informations up to date, such as lists or indexes permitting access to the filed data.
A commonly accepted solution consists of associating with the non-erasable support, an erasable support in which are stored the informations which can be subject to change. This solution suffers from two major disadvantages. Firstly the loss of informations contained in the erasable support can prevent any access to the informations contained in the non-erasable support and this is the case when the erasable support contains the lists or indexes permitting access to the filed data. Secondly as the non-erasable support can only function with the information contained in the erasable support, said two supports must be permanently associated, e.g. during transportation or storage.
Methods are also known permitting the updating of informations on the actual non-erasable disk. A first method of this type is described in European patent application EP-A1 No. 0 046 323. The main objective of he writing and reading method according to the latter is the provision of an error-free recording on a non-erasable information support organized into sectors. This method consists of writing an information into a sector and then re-reading said information immediately after writing for checking purposes. An address vector is then added to the information to be written into the following sector, so that said address vector indicates that the previously written sector contains a correct information. On re-reading, said address vector ensures that only the correctly written information is transferred to the user. Reference is also made to theproblem of updating the stored information (p 3, line 26 to p 4, line 22).
The method described makes it possible to update small information quantities, such as pointers. However, this method suffers from two major deficiencies, which are, on the one hand, that the new information is recorded to the detriment of the error correcting code, so that the complete sector can soon be made unusable, and on the other hand, said method can only be carried ou at the non-erasable support control unit, i.e. by the designer.
Another method for updating information on a non-erasable support is described in the article "Dynamic data structures on optical disc" by Peter RATHMANN, which appeared in Proceedings of the International Conference on Data Engineering, Los Angeles, Apr. 24/27 1984, pp 174 to 180. A method recommended by the author consists of associating one or more pointers with each recording. When the initial information is written in a sector, the associated pointers are left empty. When it becomes necessary to update the information contained in the sector, the new information is written into a new sector and the pointer of the sector containing the obsolete information is loaded with a value indicating the address of the sector containing the new information and the pointer of the sector associated with the new information is left empty. The same operation is repeated for successive updatings, so that a chained list of sectors is obtained, whereof the first sector contains the most recent information.
The author stresses that the greatest difficulty in this method is that it is necessary to provide an updating pointer. Thus, the non-erasable supports, such as optical discs are designed for the bulk storage of large files and are not very suitable for writing small data quantities at a random point on a sector, which is necessary for updating the pointer. Another problem results from the difficulty of bringing about a correct positioning of the write head. Finally, the writing of small data quantities is not very effective, because it tends to reduce the error correction capacity in the information writen into the sector.